security: Support ECDSA verification in all related classes.

This commit also addresses some old issues, such as "using namespace std".

Change-Id: I62b8ff97fb2b6f6cfe04b0ad5866bc0797e6e43b
Refs: #1660
diff --git a/src/security/digest-sha256.hpp b/src/security/digest-sha256.hpp
index 19833e1..6221ee6 100644
--- a/src/security/digest-sha256.hpp
+++ b/src/security/digest-sha256.hpp
@@ -47,7 +47,7 @@
   {
     m_info = Block(Tlv::SignatureInfo);
 
-    m_type = Signature::Sha256;
+    m_type = Tlv::DigestSha256;
     m_info.push_back(nonNegativeIntegerBlock(Tlv::SignatureType, Tlv::DigestSha256));
   }
 
@@ -55,7 +55,7 @@
   DigestSha256(const Signature& signature)
     : Signature(signature)
   {
-    if (getType() != Signature::Sha256)
+    if (getType() != Tlv::DigestSha256)
       throw Error("Incorrect signature type");
   }
 };